The invention relates to an integrated waveguide arrangement which includes a waveguide region, a foreign region and a temperature-adjustment unit.
Typically, the waveguide region consists of glass, for example SiO2, and includes a waveguide with glass core and glass sheath. The refractive index of the glass cores is greater than the refractive index of the glass sheath. The waveguide is such that it carries an electromagnetic wave with low losses. The foreign region consists of a material other than glass and extends in the vicinity of the waveguide. A typical distance between foreign region and waveguide is, for example, 1 micrometer (xcexcm) or 2 xcexcm.
The temperature-adjustment unit is used to heat and/or cool the foreign region, so that it is possible to influence the wave propagation in the waveguide. A typical working range of the temperature-adjustment unit is between xe2x88x9240xc2x0 C. and 150xc2x0 C.
A waveguide arrangement of this type is known form the article xe2x80x9cHybrid switches offer the best of both worldsxe2x80x9d, fiber systems, 05/2000, Vol.4, No.4, p.15, by Pauline Rigby. The wave arrangement explained in that document includes two waveguides made from glass which are connected by a waveguide made from polymer.
When producing waveguides from glass, the procedure is usually as follows:
1. An intermediate layer of silicon dioxide, which is known as a buffer layer, is applied to a silicon substrate.
2. A core layer, the refractive index of which is greater than the refractive index of the silicon dioxide layer, is applied to the silicon dioxide layer. The core layer likewise consists of silicon dioxide.
3. Regions at which there are to be no waveguide cores are removed from the core layer, usually by dry-chemical etching.
4. The remaining waveguide cores of the core layer and those areas of the intermediate layer which have been exposed in the previous process step are coated with a sheath coating of silicon dioxide which has the same refractive index as the silicon dioxide of the intermediate layer. This process is also known as cladding.
The known integrated waveguide arrangement is hybrid in the sense that it includes a waveguide made from glass and a waveguide made from polymer. Waveguides made from glass are distinguished by low transmission losses when carrying the waves. By contrast, waveguides made from polymer have a significantly higher thermo-optical coefficient than waveguides made from glass and are therefore more suitable for switching operations with the aid of the temperature-adjustment unit. The thermo-optical coefficient is a measure of the change in the refractive index as a function of the change in temperature of a material. For SiO2, the thermo-optical coefficient is approximately dn/dT=1exe2x88x926/K. On account of using both materials, the hybrid approach exploits both advantages. The result is a switching element which operates with a low switching power and low transmission losses.
It is an object of the invention to provide an integrated waveguide arrangement which is of simple structure and in which the advantages of the hybrid structure are retained. Moreover, it is intended to provide a process for producing a waveguide arrangement of this type.
The object relating to the waveguide arrangement is achieved by means of a waveguide arrangement having the features of patent claim 1. Refinements are given in the subclaims.
The invention is based on the discovery that the good properties of a polymer waveguide with regard to the high temperature coefficient are retained even if only the waveguide core consists of polymer. The waveguide sheath can be produced from glass without the transmission properties being changed significantly. Furthermore, the discovery is based on the consideration that materials other than glass, for example polymer, have a better capacity to conduct heat. When using a waveguide core made from a material other than glass in a region consisting of glass, the energy emitted by the temperature-adjustment unit can be concentrated in a smaller area, since less heat is dissipated via the waveguide sheath.
In the waveguide arrangement according to the invention, in addition to the features listed in the introduction, the foreign region is integrated or embedded in the waveguide region. The foreign region forms the region in which switching operations take place with the aid of the temperature-adjustment unit. In the waveguide arrangement according to the invention, waveguides are made from glass apart from the foreign region and therefore have low transmission losses. The foreign region itself consists of a material other than glass, which therefore has a higher thermal conductivity and a greater thermo-optical coefficient.
The integration of the foreign region in the waveguide region facilitates production of the waveguide arrangement. In addition to the process steps listed in the introduction, the following steps are carried out:
5. A trench is etched into the sheath layer.
6. The entire structure is coated with a polymer, for example by means of spinning.
Therefore, during production of the waveguide arrangement according to the invention it is simply necessary to apply a polymer layer. The manufacturing tolerances are low, because a trench of defined depth can be etched with very great accuracy.
The advantages of the hybrid approach are retained in the waveguide arrangement according to the invention. In addition, however, the effect is achieved that the heat which is dissipated by the temperature-adjustment unit is concentrated on the waveguide core made from polymer, since a glass sheath is being used. Also, during cooling operations, heat is initially extracted only from the foreign region. This leads to a reduction in the switching power required.
The refractive index of the material in the foreign region is not critical, since the high thermo-optical coefficient means that it can be set within wide limits with the aid of the temperature-adjustment unit.
The integrated waveguide arrangement according to the invention opens up the route to a large number of new types of waveguide components. Depending on the arrangement of the foreign region with regard to a glass waveguide or with regard to a plurality of glass waveguides, it is possible, inter alia, to construct switching units, coupling units, attenuators and radiators.
In a refinement of the waveguide arrangement according to the invention, the waveguide region is arranged on a planar substrate. On the side which is remote from the substrate, the waveguide region forms a surface which lies approximately parallel to the interface between substrate and waveguide region. The waveguide arrangement can be produced using the technically highly developed lithographic processes. When using the process steps 1 to 4 listed in the introduction, therefore, the waveguide region includes the intermediate layer, parts of the core layer, specifically the waveguide cores made from glass and the sheath covering. On account of flow processes during its application to the side remote from the substrate, the sheath covering has a surface which is approximately parallel to the substrate surface.
In a subsequent refinement, the foreign region is produced from a material with a thermo-optical coefficient, the magnitude of which differs significantly from the magnitude of the thermo-optical coefficient of the glass. By way of example, coefficients which in terms of magnitude are 100 times greater than the thermo-optical coefficient of glass are used. The reference temperature selected, by way of example, is room temperature, i.e. 20xc2x0 C.
In a further configuration, the material used in the foreign region is a plastic, for example, a polymer. An example of a polymer with a positive thermo-optical coefficient is a fluoroacrylate polymer which contains pentafluorostyrene (PFS), trifluoroethylmethacrylate (TFM) and glycidylmethacrylate (GMA). An example of a polymer with a negative thermo-optical coefficient is benzocyclobutene resin (RCB), which is marketed by the Dow Chemical Company under the CYCLOTENE trademark. This is a polymer which is polymerized from divinyldisiloxane-bis-benzocyclobutene monomers (B stage). The choice of polymer depends on what is required at the xe2x80x9cnormalxe2x80x9d switching state. A polymer with a thermo-optical coefficient which overall requires a lower heating capacity is selected.
In one configuration, the foreign region extends from that surface of the waveguide region which is remote from the substrate to close to the waveguide. This arrangement is a result of the production technology employed. It is possible to introduce a trench from that surface of the waveguide region which is remote from the substrate. When the material of the foreign region is then introduced into the trench, it extends as far as the edge of the trench or beyond it.
In another configuration, the waveguide core has a higher refractive index than the waveguide sheath, and the refractive index of the material in the foreign region is dependent on the temperature which is generated by the temperature-adjustment unit and, for the same waveguide arrangement, may, depending on the operating mode, be less than, equal to or greater than the refractive index of the waveguide sheath. This method of selecting refractive indices makes it possible to construct waveguide arrangements for different tasks. Examples of different components and the refractive indices required therefore are given below.
In a refinement, a damping component which operates thermo-optically is formed as a result of the foreign region extending into the waveguide core. Depending on the temperature in the foreign region, a wave is transmitted from that part of the waveguide made from glass which lies on one side of the foreign region, through the foreign region, into that part of the waveguide made from glass which lies on the other side of the foreign region. If the foreign region is at a temperature which leads to a refractive index which corresponds to the refractive index of the waveguide core made from glass, a wave is passed unchanged through the foreign region. By contrast, if the foreign region is at a temperature which leads to a refractive index which is lower than the refractive index of the glass material surrounding the foreign region, a wave cannot pass through the foreign region and is radiated into the waveguide sheath and the surrounding waveguide region. In this case, only a wave which has been attenuated to a greater or lesser extent is present in the waveguide core on the other side of the foreign region. This component is suitable for signal matching, for example upstream of a sensor element.
In other refinements, the foreign region extends approximately parallel to the glass waveguide. Since waveguide and foreign region are arranged so close together that a wave which is propagating in the waveguide is influenced, the extent of this influence is dependent both on the length of the distance over which foreign region and waveguide lie approximately parallel to one another and on the refractive index in the foreign region which is brought about by the temperature. Numerous control options and switching principles are opened up.
In a further configuration, the foreign region is arranged outside a region which lies between the glass waveguide and a substrate. When using lithographic processes to etch the trench for the foreign region, this measure does not entail any risk of the waveguide region being involuntarily destroyed, as would be the case if the trench had to be etched directly via the waveguide. In this case, the waveguide would be arranged within the region which lies between foreign region and substrate. An offset arrangement of the foreign region allows greater tolerances to be set when etching the trench.
A thermo-optical attenuator which operates according to a different principle is formed if the waveguide is curved along the direction of propagation of the waves. The foreign region is arranged at the curved regions of the waveguide. Depending on the temperature of the temperature-adjustment unit, a wave which is propagating within the glass waveguide will be drawn to a greater or lesser extent into the foreign region, from where it will be radiated into the waveguide sheath.
If, in a further configuration, glass waveguide and foreign region are of approximately the same cross section, they are suitable as waveguides for carrying the same type of wave. A wave can be transferred with relatively low transmission losses out of the glass waveguide into the foreign region or in the opposite direction. This forms the basis for numerous switching operations.
A thermo-optical reversing switch is formed in a waveguide arrangement in which the waveguide region includes two waveguides which run approximately parallel and between which the foreign region is arranged. The foreign region can be so small that, depending on the temperature, it transmits a wave transmitted in one waveguide into the other waveguide. With a small foreign region, the transmission losses in the polymer remain low. It is not necessary for the wave to be guided in the foreign region itself.
If, in a further configuration, an intermediate layer made from a material which prevents the optical field from reaching the temperature-adjustment unit is present between the temperature-adjustment unit and foreign region, waves can be transmitted with relatively low losses in the foreign region.
In an alternative configuration, however, the temperature-adjustment unit is arranged directly on the foreign region. As a result, the temperature-adjustment unit attenuates a wave which is propagating in the foreign region and therefore, in addition to emitting or absorbing heat, has a dual function. The absorption of a wave is for many applications better than diffuse radiation of the energy into the waveguide region. It is possible to construct an attenuator in which, on the one hand, the temperature-adjustment unit absorbs the wave which is propagating in the foreign region and in which, on the other hand, the material of the foreign region is selected in such a way that it has a poor transmission capacity. The drawback of the polymer, i.e. that it has higher transmission losses than a glass waveguide, can therefore be utilized as an advantage for an attenuator.
The invention also relates to a process for producing an integrated waveguide arrangement comprising the process steps given in patent claim 15. These process steps lead to the waveguide arrangement according to the invention. Therefore, the technical effects which have been listed above for the waveguide arrangement and for its configurations and refinements also apply to the process. In configurations of the process, it is modified in such a way that waveguide arrangements in accordance with the modifications referred to above are formed.
The invention relates furthermore to thermo-optical components, namely a thermo-optical radiation unit, a thermo-optical switching unit and a thermo-optical absorption unit in accordance with patent claims 16, 17 and 18, respectively. These components are closely technically related to the waveguide arrangement according to the invention and the process according to the invention.
The thermo-optical radiation unit is constructed in such a way that the foreign region interrupts the glass waveguide core. The foreign region is therefore introduced directly above the waveguide core and extends into this core or even closer to the substrate. Since the foreign region may be very small, it is possible to construct radiation units with short switching times.
The thermo-optical switching unit includes at least two waveguides which run parallel to one another at least in a section. The foreign region extends between the waveguides and lies parallel thereto. When this switching unit is being constructed, it is possible to select high manufacturing tolerances, since there is no risk of damage to the glass waveguide cores. The result is a compact thermo-optical switching unit which is simple to produce. The foreign region may be very small. This enables a switching unit with a short switching time to be produced.
In the thermo-optical absorption unit, the temperature-adjustment unit bears directly against the foreign region and absorbs a wave which is propagating in the foreign region. The temperature-adjustment unit therefore has a double function. The drawback of the material in the foreign region, namely that its transmission capacity is worse than that of glass, is in this case an advantage, since a high degree of attenuation is desired. Materials which have a high attenuation coefficient are selected for the foreign region. The thermo-optical component which is formed in this way can be used, for example, for level adapting ahead of sensor elements.